According to an internal Verizon document shared on Reddit, using Dual SIM mode on any iPhone 12 model will disable 5G and revert back to 4G LTE. The document does not mention the reason for this limitation. However, another report that MacRumors acquired says that Apple will restore 5G compatibility with Dual SIM in a future update.
Apple first introduced Dual SIM functionality with its iPhone XS lineup in 2018. It allows users to use two SIMs at the same time. However, it isn’t a traditional system. It features one physical SIM slot that you can insert your SIM into, while the other slot is embedded right into the phone. Users can add a SIM to that slot virtually.
The Verizon document also suggests users only use the e-SIM (the virtual SIM slot) to maintain a 5G connection. The iPhone 12 automatically connects to 5G given that your carrier and data plan support it and that you are under coverage area. mmWave 5G, which is a faster band of 5G, is only supported by the iPhone 12 in the US, for instance.
A major indication of that is a small groove on the right edge of the phone, which is a mmWave 5G antenna. It’s only present on the US models of the iPhone 12 lineup. As for roaming internationally, it is expected that at the moment, users should replace the pre-existing SIM on the phone with a new SIM of the country if they want to enjoy 5G speeds there.